Multiple data surface optical data storage system

ABSTRACT

An optical data storage system comprises a multiple data surface medium and optical head. The medium comprises a plurality of substrates separated by a light transmissive medium. Data surfaces are located on the substrate surfaces which lie adjacent a light transmissive medium. The data surfaces are substantially light transmissive. The optical head includes an aberration compensator to allow the head to focus onto the different data surfaces and a filter to screen out unwanted reflected light.

This is a division of application Ser. No. 07/710,226, filed Jun. 4,1991, now U.S. Pat. No. 5,255,262.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to optical data storage systems andmore specifically to a storage system having multiple data storagesurfaces.

2. Description of the Prior Art

Optical data storage systems provide a means for storing greatquantifies of data on a disk. The data is accessed by focussing a laserbeam onto the data layer of the disk and then detecting the reflectedlight beam. Various kinds of systems are known. In a ROM (Read OnlyMemory) system, data is permanently embedded as marks in the disk at thetime of manufacture of the disk. The data is detected as a change inreflectivity as the laser beam passes over the data marks. A WORM(Write-Once Read Many) system allows the user to write data by makingmarks, such as pits, on a blank optical disk surface. Once the data isrecorded onto the disk it cannot be erased. The data in a WORM system isalso detected as a change in reflectivity.

Erasable optical systems are also known. These systems use the laser toheat the data layer above a critical temperature in order to write anderase the data. Magneto-optical recording systems record data byorienting the magnetic domain of a spot in either an up or a downposition. The data is read by directing a low power laser to the datalayer. The differences in magnetic domain direction cause the plane ofpolarization of the light beam to be rotated one way or the other,clockwise or counterclockwise. This change in orientation ofpolarization is then detected. Phase change recording uses a structuralchange of the data layer itself (amorphous/crystalline are two commontypes of phases) to record the data. The data is detected as changes inreflectivity as a beam passes over the different phases.

In order to increase the storage capacity of an optical disk, multipledata layer systems have been proposed. An optical disk having two ormore data layers may in theory be accessed at different layers bychanging the focal position of the lens. Examples of this approachinclude U.S. Pat. No. 3,946,367 issued Mar. 23, 1976 by Wohlmut, et al.;U.S. Pat. No. 4,219,704 issued Aug. 26, 1980 to Russell; U.S. Pat. No.4,450,553 issued May 22, 1984 to Holster, et al.; U.S. Pat. No.4,905,215 issued Feb. 27, 1990 to Hattori, et al.; Japanese PublishedApplication, 63-276732 published Nov. 15, 1988 by Watanabe, et al.; andIBM Technical Disclosure Bulletin, Vol. 30, No. 2, p. 667, July 1987, byArter, et al.

The problem with these prior art systems has been that the ability toclearly read the data recorded is very difficult if there is more thanone data layer. The cross-talk signals from the other data layersgreatly reduces the ability to read. Also, there are problems infocussing at the different depths and in generating a tracking signal.An optical data storage system is needed which overcomes these problems.

SUMMARY OF THE INVENTION

In a preferred embodiment of the invention, an optical data storagesystem comprises an optical disk drive and a multiple data surfaceoptical medium. The medium has a plurality of substrate membersseparated by air spaces. The surfaces of the substrate members which areadjacent to the air spaces are the data surfaces. The data surfaces arehighly transmissive with the exception of the last data layer which mayinclude a reflector layer. Each data surface has tracking marks.

The disk drive comprises a laser for generating a laser beam. An opticaltransmission channel directs the light to the medium. The transmissionchannel includes a focus element for focussing the light onto thedifferent data surfaces and an aberration compensator element to correctfor aberrations due to variations in the effective substrate thickness.A reception channel receives reflected light from the medium. Thereception channel includes a filter element to screen out unwanted lightreflected from data surfaces other than the one to be read. Thereception channel has detectors for receiving the reflected light andcircuitry for generating data and servo signals responsive thereto.

For a fuller understanding of the nature and advantages of the presentinvention reference should be made to the following detailed descriptiontaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an optical data storage system of thepresent invention:

FIG. 2A is a cross-sectional view of an optical medium of the presentinvention;

FIG. 2B is a cross-sectional view of an alternative optical medium;

FIG. 3A is a cross-sectional view of the tracking marks of the medium ofFIG. 2;

FIG. 3B is a cross-sectional view of alternative tracking marks;

FIG. 3C is a cross-sectional view of alternative tracking marks;

FIG. 3D is a cross-sectional view of alternative tracking marks;

FIG. 4 is a schematic diagram of an optical head and medium of thepresent invention;

FIG. 5 is a top view of an optical detector of FIG. 4;

FIG. 6 is a circuit diagram of a channel circuit of the presentinvention;

FIG. 7 is a schematic diagram of a controller circuit of the presentinvention;

FIG. 8A is a graph of tracking error signal versus head displacement;

FIG. 8B is a graph of tracking error signal versus head displacement foran alternative embodiment;

FIG. 8C is a graph of tracking error signal versus head displacement foran alternative embodiment;

FIG. 9 is a graph of the focus error signal versus lens displacement forthe present invention;

FIG. 10 is a schematic diagram of a multiple data surface aberrationcompensator of the present invention;

FIG. 11 is a schematic diagram of an alternative embodiment of amultiple data surface aberration compensator of the present invention;

FIG. 12 is a schematic diagram of an additional alternative embodimentof a multiple data surface aberration compensator of the presentinvention;

FIG. 13 is a top view of the compensator of FIG. 12;

FIG. 14 is a schematic diagram of an additional alternative embodimentof a multiple data surface aberration compensator of the presentinvention;

FIG. 15 is a schematic diagram of an alternative embodiment of amultiple data surface aberration compensator of the present invention;

FIG. 16 is a cross-sectional view of the lens of FIG. 15;

FIG. 17 is a schematic diagram of an alternative embodiment of anoptical head and medium of the present invention;

FIG. 18 is a schematic diagram of an alternative embodiment of amultiple data surface aberration compensator of the present invention;

FIG. 19 is a schematic diagram of an alternative embodiment of amultiple data surface aberration compensator of the present invention;

FIG. 20 is a schematic diagram showing the process of manufacturing thecompensator of FIGS. 18 and 19;

FIG. 21 is a schematic diagram of an alternative embodiment of theaberration compensator of the present invention;

FIG. 22 is a schematic diagram of an alternative embodiment of theaberration compensator of the present invention;

FIG. 23 is a schematic diagram of a multiple data surface filter of thepresent invention;

FIG. 24 is a schematic diagram of an alternative embodiment of amultiple data surface filter of the present invention;

FIG. 25 is a schematic diagram of an alternative embodiment of amultiple data surface filter of the present invention; and

FIG. 26 is a schematic diagram showing the process of manufacturing thefilter of FIG. 25.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present application is related to concurrently filed U.S. Patentapplications "Multiple Data Surface Optical Data Storage System andMethod" by H. Rosen, K. Rubin, G. Sincerbox, T. Strand and J. Zavislan,and "Multiple Data Surface Data Storage System and Method" by H. Rosen,K. Rubin and T. Strand.

FIG. 1 shows a schematic diagram of an optical data storage system ofthe present invention and is designated by the general reference number10. System 10 includes an optical data storage medium 12 which ispreferably disk shaped. Medium 12 is removably mounted on a clampingspindle 14 as is known in the art. Spindle 14 is attached to a spindlemotor 16 which in turn is attached to a system chassis 20. Motor 16rotates spindle 14 and medium 12.

An optical head 22 is positioned below medium 12. Head 22 is attached toan arm 24 which in turn is connected to an actuator device, such as avoice coil motor 26. Voice coil motor 26 is attached to chassis 20.Motor 26 moves arm 24 and head 22 in a radial direction below medium 12.

THE OPTICAL MEDIUM

FIG. 2A is a cross-sectional view of medium 12. Medium 12 has asubstrate 50. Substrate 50 is also known as the face plate or coverplate and is where the laser beam enters medium 12. An outer diameter(OD) rim 52 and an inner diameter (ID) rim 54 are attached between faceplate 50 and a substrate 56. An OD rim 58 and an ID rim 60 are attachedbetween substrate 56 and a substrate 62. An OD rim 64 and an ID rim 66are attached between substrates 62 and a substrate 68. An OD rim 70 andID rim 72 are attached between substrates 68 and a substrate 74. Faceplate 50 and substrates 56, 62, 68 and 74 are made of a lighttransmissive material such as glass, polycarbonate or other polymermaterial. In a preferred embodiment, face plate 50 is 1.2 mm thick andsubstrates 56, 62, 68 and 74 are 0.4 mm thick. The substrate mayalternatively have thicknesses of 0.2 to 0.8 mm. The ID and OD rims arepreferably made of a plastic material and are approximately 500 micronsthick. The rims may alternatively have thicknesses of 50-500 microns.

The rims may be attached to the face plate and substrates by means ofglue, cement or other bonding process. The rims may alternatively beintegrally formed in the substrates. When in place, the rims form aplurality of annular spaces 78 between the substrates and the faceplate. A spindle aperture 80 passes through medium 12 inside the ID rimsfor receiving the spindle 14. A plurality of passages 82 are provided inthe ID rims connecting the aperture and the spaces 78 to allow pressureequalization between the spaces 78 and the surrounding environment ofthe disk file, which would typically be air. A plurality of lowimpedance filters 84 are attached to passages 82 to preventcontamination of spaces 78 by particulate matter in the air. Filters 84may be quartz or glass fiber. Passages 82 and filters 84 couldalternatively be located on the OD rim.

Surfaces 90, 92, 94, 96, 98, 100, 102 and 104 are data surfaces and lieadjacent spaces 78. These data surfaces may contain ROM data which isformed directly into the substrate surfaces or, alternatively the datasurfaces may be coated with one of the various writeable optical storagefilms such as WORM, or one of the various erasable optical storage filmssuch as phase change, or magneto-optical. Other than the optical storagefilms themselves, the data surfaces are made without the separatemetallic reflector layer structures (reflectivity from 30-100%) whichare known in the prior art such as U.S. Pat. No. 4,450,553. In otherwords, the data surfaces may comprise, consist of or essentially consistof the surface itself in the case of a ROM surface or the surface and anoptical storage film in the case of WORM, phase change or magneto-opticsurfaces. An additional nondata storing reflector layer is not needed.The result is that the data surfaces are very light transmissive andmany data surfaces are possible. Although the intermediate data surfacesdo not have reflector layers, a reflector layer may optionally be addedbehind the last data surface 104 to achieve greater reflection from thelast data surface 104.

In the preferred embodiment, the data surfaces are ROM surfaces. Data ispermanently recorded as pits which are formed directly into thesubstrate at the time the disk is manufactured. In contrast to the priorart, the ROM surfaces of the present invention do not have metallicreflector layers. The substrates have no coatings. The result is thatthe transmissivity of each data surface is approximately 96%. The 4%reflectivity is sufficient to detect the data. The high transmissivityhas the benefit of allowing a large number of data surfaces to beaccessed and minimizes the effects of unwanted signals from othersurfaces. Since there are no coatings on these surfaces, they are easierto manufacture and are more resistant to corrosion.

Although it is not necessary, it may be desirable to increase thereflectivity to reduce the required laser power. One way to increase thereflectivity above 4% is to apply a thin film coating of a dielectricwhich has an index of refraction greater than the substrate. The maximumreflectivity of 20% occurs at a dielectric thickness of approximatelyλ/4 n, and varies monotonically to a minimum reflectivity of 4% at athickness of approximately λ/2 n, where λ is the wavelength of the lightand n is the index of refraction of the dielectric. Examples of suchdielectric materials are ZrO₂, ZnS, SiNx or mixed oxides. The dielectricmay be deposited by sputtering as is known in the art.

The reflectivity of the data layer can also be reduced below 4%. Thisincreases the transmittance and allows more disks to be stacked. Thereduction in reflectivity occurs when a dielectric film which has anindex of refraction less than the substrate is used. One such dielectricis MnF which has a index of refraction of 1.35. The minimum reflectivityof 1% occurs at a dielectric thickness of approximately λ/4 n, andvaries monotonically to a maximum reflectivity of 4% at a thickness ofapproximately λ/2n, where λ is the wavelength of the light and n is theindex of refraction. There are many other thin film anti-reflectionmaterials which could also be used. These anti-reflection films may beapplied by sputtering processes as are known in the art.

The data surfaces may alternatively contain WORM data. WORM films suchas tellurium-selenium alloys or phase change WORM films may be coatedonto the data surfaces. The films are vacuum deposited by sputtering orevaporation onto the substrate as is known in the art. The amount ofreflection, absorption, and transmission of each film is related to itsthickness and optical constants. In a preferred embodiment,tellurium-selenium alloy is deposited at a thickness of 20-800Angstroms.

The data surface may alternatively contain reversible phase changefilms. Any type of phase change films may be used, however, preferredcompositions include those that lie along or close to the tielineconnecting GeTe and Sb₂ Te₃, which include Te₅₂.5 Ge₁₅.3 Sb₃₃, Ge₂ Sb₂Te₅, GeSb₂ Te₄ and GeSb₄ Te₇. The films are vacuum deposited bysputtering processes as are known in the art onto the substrate to athickness between 20-800 Angstroms. An optional protective overcoat of3,000 Angstroms of dielectric may be formed on top of the phase changefilm in order to help prevent ablation.

Data surfaces may also alternatively contain magneto-optical films.Magneto-optical film such as rare earth transition metals are vacuumdeposited by sputtering processes as are known in the art onto thesubstrate to a thickness of 20-800 Angstroms.

A further alternative is to have the data surfaces contain a combinationof ROM, WORM, or erasable media. The higher transmission surfaces suchas ROM are preferably located closer to the light source and the lowertransmission surfaces such as WORM, phase change and magneto-optical arepreferably located furthest away. The dielectric and anti-reflectionfilms described above with the ROM surface may also be used with WORMand erasable media.

FIG. 2B is a cross-sectional view of an alternative embodiment of anoptical recording medium and is designated by the general referencenumber 120. Elements of medium 120 which are similar to elements ofmedium 12 are designated by a prime number. Medium 120 does not have therims and spaces 78 of medium 12. Instead, a plurality of solidtransparent members 122 separates the substrates. Members 122 are madeof a material having a different index of refraction than thesubstrates. This is necessary to achieve some reflection at the datasurfaces. In a preferred embodiment, the members 122 are made of anoptical cement which also serves to hold the substrate together. Thethickness of members 122 is preferably approximately 100-300 microns.Medium 120 may be substituted for medium 12 in system 10.

FIG. 3A shows an exaggerated detailed cross-sectional view of apreferred data surface pattern of medium 12 and is designated by thegeneral reference number 130. Surface 90 contains a pattern of spiral(or alternatively concentric) tracking grooves 132. The portions ofsurface 90 located between the grooves 132 are known as the landportions 134. Surface 92 contains a pattern of spiral inverse trackinggrooves (raised ridges) 136. The portion of surface 92 located betweenthe inverse grooves 136 is the land 138. The grooves 132 and the inversegrooves 136 are also referred to as tracking marks. In a preferredembodiment, the widths 140 of the tracking marks are 0.6 microns and thewidth 142 of the land sections is 1.0 microns. This results in a pitchof (1.0+0.6)=1.6 microns.

The tracking marks are used to keep the light beam on track while themedium 12 rotates. This is described in more detail below. For pattern130, a beam 144 from the optical head 22 will track on the land portion134 or 138 depending upon which surface it is focussed upon. Therecorded data is on the land portions. In order for the tracking errorssignal (TES) to be of equal magnitude for both surfaces 90 and 92 theoptical path difference between light reflected from the lands andtracking marks must be the same for both surfaces. Beam 144 focuses onsurface 90 through substrate 50, however, beam 144 focuses on surface 92through space 78. In the preferred embodiment space 78 contains air. Forthe optical path length difference between the lands and tracking marksto be the same d1n1 must equal d2n2 (or d2/d1 equals n1/n2), where d1 isthe depth of mark 132 (perpendicular distance), n1 is the index ofrefraction of substrate 50, d2 is the height of mark 136 (perpendiculardistance), and n2 is the index of refraction of space 78. In a preferredembodiment, space 78 contains air which has an index of refraction of1.0 and substrate 50 (as well as the other substrates) has an index ofrefraction 1.5. So the ratio of d2/d1 equals 1.5. In a preferredembodiment, d1 is 700 Angstroms and d2 is 1050 Angstroms. The samepattern of tracking marks is repeated on the other surfaces of medium12. The other substrate incident surfaces 94, 98 and 102 are similar tosurface 90 and the other space incident surfaces 96, 100 and 104 aresimilar to surface 92.

Although the tracking marks are preferably arranged in a spiral pattern,they may alternatively be in a concentric pattern. In addition, thespiral pattern may be the same for each data surface, i.e., they are allclockwise or counter-clockwise spirals, or they may alternate betweenclockwise and counter-clockwise spiral patterns on consecutive datalayers. This alternating spiral pattern may be preferable for certainapplications, such as storage of video data, movies for example, wherecontinuous tracking of data is desired. In such a case, the beam tracksthe clockwise spiral pattern inward on the first data surface until thespiral pattern ends near the inner diameter, and then the beam isrefocused on the second data surface directly below and then the beamtracks the counter-clockwise spiral pattern outward until the outerdiameter is reached.

FIG. 3B shows an exaggerated detailed cross-sectional view of analternative surface pattern for medium 12 and is designated by thegeneral reference number 150. Pattern 150 is similar to pattern 130except that the tracking marks for surface 92 are grooves 152 instead ofinverse grooves. The pitch and the ratio of d2/d1 are the same as forpattern 130. Beam 144 will track on land 134 on surface 90, but now beam144 will track on groove 152 when focussed on surface 92. Tracking inthe groove 132 may be desirable in certain situations. However, as willbe described below, beam 144 may also be electronically controlled totrack on land 138 of surface 92. The tracking marks for surfaces 94, 98and 102 are similar to surface 90 and the surfaces 96, 100 and 104 aresimilar to surface 92.

FIG. 3C shows an exaggerated detailed cross-sectional view of analternative surface pattern for medium 12 which is designated by thegeneral reference number 160. Pattern 160 is similar to pattern 130except that surface 90 has inverse grooves 162 instead of grooves 132,and surface 92 has grooves 164 instead of inverse grooves 136. The pitchand ratio of d2/d1 are the same as for pattern 130. Beam 144 will trackon inverse grooves 162 when focussed on surface 90 and will track ongrooves 164 when focussed on surface 92(unless it is electronicallyswitched to track on the land). The pattern for surfaces 94, 98 and 102are similar to surface 90 and the surfaces 96, 100 and 104 are similarto surface 92.

FIG. 3D shows an exaggerated detailed cross-sectional view of analternative surface pattern designated by the general reference number170. In pattern 170, the surface 90 has a similar structure to surface90 of pattern 160. Surface 92 has a similar structure to surface 92 ofpattern 130. The pitch and ratio of d2/d1 is the same as for pattern130. Beam 144 will track on inverse grooves 162 when focussed on surface90 (unless it is electronically switched to track on the land) and willtrack on land 138 when focussed on surface 92. Surfaces 94, 98 and 102have similar patterns to surface 90 and surfaces 96, 100 and 104 havepatterns similar to surface 92.

For all of the patterns 130, 150, 160 and 170 the tracking marks areformed into the substrate at the time of manufacture by injectionmolding or photopolymer processes as are known in the art. It should benoted that the optical films, as described above, are deposited onto thesubstrates after the tracking marks are formed.

The discussion of tracking marks is also applicable to other features ofoptical disks. For example, some ROM disks use pits embossed in thesubstrate to record data and/or provide tracking information. Otheroptical media use pits to emboss sector header information. Some mediause these header pits to also provide tracking information. In usingsuch media in the multiple data surface form of the present invention,the pits are formed as pits or inverse pits on the various data surfacescorresponding in a similar manner to the tracking marks discussed above.The optical path length between the lands and the pits or inverse pitsis also similar to the tracking marks. The pits, inverse pits, groovesand inverse grooves are all located at a different elevation from theland (i.e. the perpendicular distance between them and the land), andare all referred to as marks for purposes of this discussion. Markswhich are specifically dedicated to providing tracking information areknown as nondata tracking marks.

THE OPTICAL HEAD

FIG. 4 shows a schematic diagram of an optical head 22 and medium 12.Optical head 22 has a laser diode 200. Laser 200 may be agallium-aluminum-arsenide diode laser which produces a primary beam oflight 202 at approximately 780 nanometers wavelength. Beam 202 iscollimated by lens 203 and is circularized by a circularizer 204 whichmay be a circularizing prism. Beam 202 passes to a beamsplitter 205. Aportion of beam 202 is reflected by beamsplitter 205 to a focus lens 206and an optical detector 207. Detector 207 is used to monitor the powerof beam 202. The rest of beam 202 passes to and is reflected by a mirror208. Beam 202 then passes through a focus lens 210 and a multiple datasurface aberration compensator 212 and is focused onto one of the datasurfaces (surface 96 as shown) of medium 12. Lens 210 is mounted in aholder 214. The position of holder 214 is adjusted relative to medium 12by a focus actuator motor 216 which may be a voice coil motor.

A portion of the light beam 202 is reflected at the data surface as areflected beam 220. Beam 220 returns through compensator 212 and lens210 and is reflected by mirror 208. At beamsplitter 205, beam 220 isreflected to a multiple data surface filter 222. The beam 220 passesthrough filter 222 and passes to a beamsplitter 224. At beamsplitter 224a first portion 230 of beam 220 is directed to an astigmatic lens 232and a quad optical detector 234. At beamsplitter 224 a second portion236 of beam 220 is directed through a half-wave plate 238 to apolarizing beamsplitter 240. Beamsplitter 240 separates light beam 236into a first orthogonal polarized light component 242 and a secondorthogonal polarized light component 244. A lens 246 focuses light 242to an optical detector 248 and a lens 250 focuses light 244 to anoptical detector 252.

FIG. 5 shows a top view of a quad detector 234. The detector 234 isdivided into four equal sections 234A, B, C and D.

FIG. 6 shows a circuit diagram of a channel circuit 260. Circuit 260comprises a data circuit 262, a focus error circuit 264 and a trackingerror circuit 266. Data circuit 262 has an amplifier 270 connected todetector 248 and an amplifier 272 connected to detector 252. Amplifiers270 and 272 are connected to a double pole, double throw electronicswitch 274. Switch 274 is connected to a summing amplifier 276 and adifferential amplifier 278.

Circuit 264 has a plurality of amplifiers 280, 282, 284 and 286connected to detector sections 234A, B, C and D, respectively. A summingamplifier 288 is connected to amplifiers 280 and 284, and a summingamplifier 290 is connected to amplifiers 282 and 286. A differentialamplifier 292 is connected to summing amplifiers 288 and 290.

Circuit 266 has a pair of summing amplifiers 294 and 296, and adifferential amplifier 298. Summing amplifier 294 is connected toamplifiers 280 and 282, and summing amplifier 296 is connected toamplifiers 284 and 286. Differential amplifier 298 is connected tosumming amplifiers 294 and 296 via a double pole double throw electronicswitch 297. Switch 297 acts to invert the inputs to amplifier 298.

FIG. 7 is a schematic diagram of a controller system of the presentinvention and is designated by the general reference number 300. A focuserror signal (FES) peak detector 310 is connected to the focus errorsignal circuit 264. A track error signal (TES) peak detector 312 isconnected to the tracking error signal circuit 266. A controller 314 isconnected to detector 310, detector 312, detector 207 and circuits 262,264 and 266. Controller 314 is a microprocessor based disk drivecontroller. Controller 314 is also connected to and controls the laser200, head motor 26, spindle motor 16, focus motor 216, switches 274 and297, and compensator 212. The exact configuration and operation ofcompensator 212 is described in more detail below.

The operation of system 10 may now be understood. Controller 314 causesmotor 16 to rotate disk 12 and causes motor 26 to move head 22 to theproper position below disk 12. See FIG. 4. Laser 200 is energized toread data from disk 12. The beam 202 is focussed by lens 210 on the datasurface 96. The reflected beam 220 returns and is divided into beams230, 242 and 244. Beam 230 is detected by detector 234 and is used toprovide focus and tracking servo information, and beams 242 and 244 aredetected by detectors 248 and 252, respectively, and are used to providedata signals.

See FIG. 5. When beam 202 is exactly focussed on data surface 96, beam230 will have a circular cross-section 350 on detector 234. This willcause circuit 264 to output a zero focus error signal. If beam 202 isslightly out of focus one way or the other, beam 230 will fall as anoval pattern 352 or 354 on detector 234. This will cause circuit 264 tooutput a positive or negative focus error signal. Controller 314 willuse the focus error signal to control motor 216 to move lens 210 untilthe zero focus error signal is achieved.

If beam 202 is focussed exactly on a track of data surface 96, then beam230 will fall as a circular cross-section 350 equally between thesections A and B, and the sections D and C. If the beam is off track itwill fall on the boundary between a tracking mark and the land. Theresult is that the beam is diffracted and cross-section 350 will move upor down. More light will be received by sections A and B, and less bysections C and D or vice versa.

FIG. 8A shows a graph of the TES produced by circuit 264 versus thedisplacement of head 22. Controller 314 causes VCM 26 to move head 22across the surface of medium 12. TES peak detector 312 counts the peaks(maximum and minimum points) of the TES signals. There are two peaksbetween each track. By counting the number of peaks, controller 314 isable to position the beam on the proper track. The TES signal at a landis a positive slope TES signal. Controller 314 uses this positive slopesignal to lock the beam on track. For example, a positive TES signalcauses head 22 to move to the left toward the zero point land positionand a negative TES signal causes the head 22 to move to the right towardthe zero point land position. FIG. 8A is the signal derived from thepreferred pattern 130 of medium 12 when switch 297 is in its initialposition as shown in FIG. 6. The same signal is also generated forsurface 90 of pattern 150, and surface 92 of pattern 170. The beam isautomatically locked to the land because that is the position wherethere is a positive slope.

FIG. 8B shows a graph of the TES versus head displacement for surface 92of pattern 150, surfaces 90 and 92 of pattern 160 and surface 90 ofpattern 170 when switch 297 is in its initial position. Note that inthis case the tracking marks are such that the positive slope signaloccurs at the location of the tracking marks and so that the beam willautomatically track on the tracking marks and not the land portions.Tracking on the tracking marks may be desirable in some circumstances.

FIG. 8C shows a graph of the TES versus head displacement for surface 92of pattern 150, surfaces 90 and 92 of pattern 160 and surface 90 ofpattern 170 when inverter switch 297 is enabled such that the TES signalis inverted. The TES now has a positive slope at the land positions andthe beam will track on the land portion instead of the tracking marks.Thus, controller 314 can track the grooves or the lands by settingswitch 297.

In the preferred embodiment, medium 12 contains ROM data surfaces.Reflectivity detection is used to read the ROM data. In data circuit262, switch 274 is positioned to connect amplifier 276 when a ROM diskis to be read. The signal from detectors 248 and 252 is added. Lesslight is detected where data spots have been recorded and thisdifference in light detected is the data signal. Switch 274 will havethe same setting for reading WORM and phase change data disk. If disk 12has magneto-optical data surfaces, then polarization detection is neededto read the data. Switch 274 will be set to connect amplifier 278. Thedifference in the orthogonal polarization light detected at detectors248 and 252 will then provide the data signal.

FIG. 9 shows a graph of the focus error signal from circuit 264 versusthe displacement distance of lens 210. Note that a nominally sinusoidalfocus error signal is obtained for each of the data surfaces of medium12. Between the data layers, the focus error signal is zero. Duringstartup of the system, controller 314 first causes motor 216 to positionlens 210 at its zero displacement position. Controller 314 will thenseek the desired data surface by causing motor 216 to move lens 210 in apositive displacement direction. At each data layer, peak detector 310will detect the two peaks of the focus error signal. Controller 314 willcount the peaks (two per data surface) and determine the exact datasurface on which beam 202 is focussed. When the desired surfaces arereached, controller 314 causes motor 216 to position lens 210 such thatthe focus error signal is between the two peaks for that particular datasurface. The focus error is then used to control the motor 216 to seekthe zero point focus error signal between the peaks, i.e. lock on thepositive slope signal such that exact focus is achieved. The controller314 will also adjust the power of laser 200, the switch 297, and theaberration compensator 212 as appropriate for that particular datasurface.

Also on startup, controller 314 determines what type of disk it isreading. Switch 274 is first positioned for reflectivity detection andswitch 297 is set to read the land portions of the disk of the preferredpattern 130. The controller 314 seeks and reads the header informationof the first track of the first data surface. The header has informationon the number of layers, what type of optical media is in each layer(reflectivity or polarization detection), and what type of tracking markpatterns are used. With this information, the controller 314 is able toset switches 274 and 297 to correctly read each data surface. Forexample, the disk may have four layers of ROM data surfaces and twolayers of MO data surfaces. Controller 314 will set switch 274 toreflectivity detection for surfaces 1-4 and to polarization detectionfor surfaces 5-6.

If controller 314 is unable to read the first track of the first datasurface (perhaps the first layer has a different tracking mark pattern),then controller 314 will set switch 297 to its other setting and willattempt to read the first track of the first data surface again. If thisstill does not work (perhaps the first data surface is magneto-optic andrequires polarization detection) then the controller will set switch 274to the polarization detection and try again, setting switch 297 at onesetting and then the other. In summary, controller 314 will read theheader information of the first track of the first data surface bytrying the four different combinations of settings of switches 274 and297 until it is successful at reading the track. Once controller 314 hasthis header information, it can correctly set the switches 274 and 297for each of the other data surfaces.

Alternatively, the disk drive may be specifically dedicated to work withonly one type of medium. In that case, controller 314 is preprogrammedto store information on the type of data surfaces, number of layers, andtypes of tracking marks.

THE ABERRATION COMPENSATOR

Lenses are typically designed to focus light through air which has anindex of refraction of 1.0. When such lenses focus light throughmaterials having different indices of refraction, the light experiencesa spherical aberration, which distorts and enlarges the beam spot,degrading the reading and recording performance.

In typical optical data storage systems, there is only one data surfaceonto which to focus. The data surface is usually located beneath a 1.2mm thick face plate. The lens is typically a 0.55 numerical aperture(NA) lens which is specially designed to correct for sphericalaberration caused on the light by the 1.2 mm face plate. The result isthat a good spot focus can be obtained at that exact depth, but at otherdepths the focus gets blurry. This causes severe problems for anymultiple data layer system.

The aberration compensator 212 of the present invention solves thisproblem. FIG. 10 shows a schematic diagram of an aberration compensatorwhich is designated by the general reference number 400 and may be usedas compensator 212. Compensator 400 comprises a stepped block 402 havingthree steps. A first step 404 has a thickness of 0.4 mm, a second step406 has a thickness of 0.8 mm and a third step 408 has a thickness of1.2 mm. The block 402 is made of the same material as the face plate andsubstrates of medium 12 or other similar optical material. Note thatthese steps increase in optical thickness in increments of the substratethickness. Block 402 is attached to a voice coil motor 410 (or similaractuator device) which in turn is connected to controller 314. Motor 410moves block 402 laterally into and out of the path of beam 302.

Lens 210 is designed to focus on the lowest data surface of medium 12.In other words, lens 210 is designed to compensate for sphericalaberrations caused by the combined thicknesses of the face plate and theintervening substrates. For the present invention, in order to focus onsurface 102 or 104, beam 202 must pass through the face plate 50 andsubstrates 56, 62 and 68 (a combined thickness of 2.4 mm of thesubstrate material). Note that the air spaces 78 are not counted becausethey impart no additional spherical aberration. Lens 210 is thusdesigned to focus through 2.4 mm of polycarbonate and may focus equallywell on both data surfaces 102 and 104.

When beam 202 is focussed on either surface 102 or 104, the block 402 iscompletely withdrawn and beam 202 does not pass through it. When beam202 is focussed on surface 98 or 100, block 402 is positioned such thatbeam 202 passes through step 404. When beam 202 is focussed on surfaces94 or 96, block 402 is positioned such that beam 202 passes through step406. When beam 202 is focussed on surfaces 90 or 92, block 402 ispositioned such that beam 202 passes through step 408. The result isthat no matter which pair of surfaces are focussed on, beam 202 willalways pass through the same total optical thickness of material andwill not experience spherical aberration problems. Controller 314controls motor 410 to move the block 402 as appropriate.

FIG. 11 shows an aberration compensator which is designated by thegeneral reference number 430 and which may be used for compensator 212.Compensator 430 has a pair of complementary triangular shaped blocks 432and 434. Blocks 432 and 434 are made of the same material as face plateand substrates of medium 12 or material of similar optical properties.Block 432 is positioned in a fixed position such that beam 202 passesthrough it. Block 434 is attached to a voice coil motor 436 and may beslid along the surface of block 432. Controller 314 is connected to andcontrols motor 436. By moving block 434 relative to block 432 theoverall thickness of material through which beam 202 passes may beadjusted. The result is that beam 202 passes through the same opticalthickness of material no matter which data surface it is focussed on.

FIGS. 12 and 13 show an aberration compensator which is designated bythe general reference number 450 and may be used for compensator 212.Compensator 450 has a circular stepped element 452. Element 452 has foursections 454, 456, 458 and 460. Sections 456, 458 and 460 havethicknesses similar to steps 404, 406 and 408, respectively, ofcompensator 400. Section 454 has no material and represents a blankspace in the circular pattern as shown in FIG. 13. The circular element452 is attached to a stepper motor 462 which in turn is controlled bycontroller 314. Spindle 462 rotates element 452 such that beam 202passes through the same thickness of material no matter which datasurface it is focussed on.

FIG. 14 shows an aberration compensator which is designated by thegeneral reference number 570 and may be used for compensator 212.Compensator 570 comprises a stationary convex lens 572 and a moveableconcave lens 574. Lens 574 is attached to a voice coil motor 576. Voicecoil motor 576 is controlled by controller 314 to move lens 574 relativeto lens 572. Beam 202 passes through lens 572, lens 574 and lens 210 tomedium 12. Moving lens 574 relative to lens 572 changes the sphericalaberration of beam 202 and allows it to focus on the different datasurfaces. In a preferred embodiment lenses 210, 574 and 572 comprise aCooke triplet having movable center element 574. Cooke triplets aredescribed in more detail in the article by R. Kingslake, "Lens DesignFundamentals," Academic Press, New York, 1978, pp. 286-295. Althoughlens 574 is shown as the moving element, alternatively, lens 574 couldbe stationary and lens 572 used as the moving element. In FIG. 4 theaberration compensator 212 is shown between lens 210 and medium 12.However, if compensator 570 is used it will be located between lens 210and mirror 208 as shown in FIG. 14.

FIG. 15 shows an aberration compensator which is designated by thegeneral reference number 580. Compensator 580 comprises an aspheric lenselement 582 with nominally zero focal power. Element 582 has a sphericalaberration surface 584 and a planar surface 586. Lens 582 is connectedto a voice coil motor 588. Voice coil motor 588 is controlled bycontroller 314 which moves lens 582 relative to lens 512. Beam 202passes through lens 210 and lens 582 to medium 12. Moving lens 582relative to lens 210 changes the spherical aberration of the beam 202and allows it to focus on the different data surfaces.

FIG. 16 shows a view of lens 582 relative to axes z and ρ. In apreferred embodiment, the surface of 584 should correspond to theformula

    Z=0.00770ρ.sup.4 -0.00154ρ.sup.6.

FIG. 17 shows a schematic diagram of an alternative optical head of thepresent invention and is designated by the general reference number 600.Elements of head 600 which are similar to elements of head 22 aredesignate by a prime number. Note that head 600 is similar to system 10except that the aberration compensator 212 has been eliminated and a newaberration compensator 602 has been added between beamsplitter 206' andmirror 208'. The description and operation of compensator 602 isdescribed below. The operation of head 600 is otherwise the same asdescribed for head 22. Head 600 may be substituted for head 22 in system10.

FIG. 18 shows a schematic diagram of an aberration compensator which isdesignated by the general reference number 610 and may be used forcompensator 602. Compensator 610 comprises a substrate 612 having areflective holographic coating 614. Substrate 612 is attached to astepper motor 616 which in turn is controlled by controller 314.Holographic coating 614 has a number of different holograms recorded,each of which imparts a particular spherical aberration to beam 202'.These holograms are of the Bragg type which are sensitive only to lightincident at a specific angle and wavelength. When substrate 612 isrotated a few degrees, beam 202' will experience a different hologram.The number of holograms recorded corresponds to the number of differentspherical aberration corrections required. For medium 12 as shown, fourdifferent recordings are necessary each corresponding to one of thepairs of data surfaces.

FIG. 19 shows a schematic diagram of an aberration compensator which isdesignated by the general reference number 620 and may be used forcompensator 602. Compensator 620 comprises a substrate 622, atransmissive holographic coating 624 and a stepper motor 626. Thecompensator 620 is similar to compensator 610 except that here theholographic coating 624 is transmissive rather than reflective.Holographic coating 624 has a number of holograms recorded, each ofwhich corresponds to the amount of spherical aberration compensationrequired. Beam 202' experiences each of these holograms in turn assubstrate 622 is rotated.

FIG. 20 shows a schematic diagram of a recording system used to make theholographic coatings 614 and 624, and is designated by the generalreference number 650. System 650 has a laser 652 which produces a lightbeam 654 at a frequency similar to the laser 200. Light 654 iscollimated by lens 656 and is passed to a beamsplitter 658. Beamsplitter658 divides the light into a beam 660 and a beam 662. Beam 660 isreflected by a mirror 664 and 666, and is focussed by a lens 668 to apoint 670 in a plane 672. Beam 660 passes through a stepped block 674similar to block 402. Beam 660 is then recollimated by a lens 676 andfalls upon a holographic coating 680 on a substrate 682. Substrate 682is rotatably mounted to a stepper motor 684. Beam 662 also falls uponcoating 680 at a 90 degree angle from beam 660.

Lens 668 forms an unaberrated spot on plane 672. This light is thenpassed through a step of block 674 which has a thickness representingthe sum of the substrate thicknesses which will be encountered inaccessing a particular recording layer. Lens 676 is identical in designto lens 210 as used in the optical storage head. It collimates the lightinto a beam that contains a specific amount of spherical aberrationcorresponding to the specific thickness. This wavefront isholographically recorded by interference with the reference beam 662. Ifthe hologram is oriented in approximately a plane 690 as shown, atransmission hologram is recorded. If it is oriented in approximately aplane 692 as shown as a dash line, a reflective hologram is recorded.The wavefront required to correct the aberrations encountered inaccessing a different pair of recording layers is holographically storedby rotating the hologram to a new angular position and inserting thecorresponding thickness plate of block 674. A multiplicity of angularlyresolved holograms are recorded, each corresponding to and providingcorrection for a different pair of recording layers. The holographiccoating may be made of dichromated gelatin or a photopolymer material.The individual holograms can be recorded in increments as small as onedegree without appreciable cross-talk. This permits large numbers ofholograms to be recorded and correspondingly large numbers of datasurfaces to be used.

FIG. 21 shows a schematic diagram of an alternative aberrationcompensator which is designated by the general reference number 700 andmay be used for compensator 602. Compensator 700 comprises a polarizingbeamsplitter 702, a quarter waveplate 704, a carousel 706 attached to astepper motor 708 and a plurality of spherical aberration mirrors 710each providing a different spherical aberration correction. Beam 202' isoriented with its polarization such that it passes through beamsplitter702 and plate 704 to one of mirrors 710. Mirror 710 imparts theappropriate spherical aberration to the beam 202' which then returnsthrough plate 704 and is reflected by beamsplitter 702 to mirror 208'.Motor 708 is controlled by controller 314 to rotate the carousel 706 toposition the appropriate mirror in place. Mirrors 710 are reflectingSchmidt corrector plates. See M. Born, et al., "Principles of Optics,"Pergonan Press Oxford, 1975, pp. 245-249.

FIG. 22 shows a schematic diagram of an aberration compensator which isdesignated by the general reference number 720 and may be used forcompensator 602. Compensator 720 comprises a polarizing beamsplitter722, a quarter waveplate 724 and an electrical controlled deformablemirror 726. Deformable mirror 726 is controlled by internalpiezo-electric elements and is described in more detail in J. P.Gaffarel, et al., "Applied Optics," Vol. 26, pp. 3772-3777, (1987). Theoperation of compensator 720 is similar to compensator 700, except thatmirror 726 is electrically adjusted to provide the appropriate sphericalaberration. In other words, mirror 726 is adjusted to form a reflectivesurface corresponding to the different Schmidt corrector plates 710 ofcompensator 700. Controller 314 controls the adjustment of mirror 726 asappropriate.

The operation of the aberration compensators 212 and 602 have beendescribed above in connection with medium 12. Due to the air spacebetween the layers, one aberration compensation setting will work foreach pair of data surfaces. However, in the case where medium 120 isused, aberration compensation settings will need to be made for eachdata surface. This is because there are no air spaces.

MULTIPLES DATA SURFACE FILTER

When beam 202 is focussed on a particular data surface of medium 12 areflected beam 230 is returned to head 22 from that surface. However,some of light beam 202 is also reflected at the other data surfaces.This unwanted reflected light must be screened out for proper data andservo signals to be obtained. The multiple data surface filter 222 ofthe present invention achieves this function.

FIG. 23 shows a schematic diagram of a filter 750 which may be used asfilter 222. Filter 750 comprises a blocking plate 754 and a lens 756.The desired light beam 230 is collimated because it is the light whichhas been properly focussed by lens 210. Beam 230 is focussed by lens 752to a point 760. Unwanted light 762 is not properly focussed by lens 210and is thus not collimated. The light 762 will not focus to point 760.Plate 764 has an aperture 764 at point 760 which allows light 230 topass. Most of the unwanted light 762 is blocked by plate 754. The light230 is recollimated by lens 756. In a preferred embodiment aperture 764is circularly shaped and has a diameter of approximately λ/(2*(NA)),where λ is the wavelength of the light and N.A. is the numericalaperture of lens 752. The exact diameter is determined by the desiredtrade-off between alignment tolerances and interlayer signal rejectionrequirements. Alternatively, aperture 764 may be a slit having a minimumgap distance of approximately λ/(2*(NA)). In such a case plate 764 couldbe two separate members which are separated by the slit. Plate 754 maybe made of a metal sheet or may be made of a transparent substratehaving a light blocking coating with aperture 764 being uncoated.

FIG. 24 shows a schematic diagram of a filter 800 which also may be usedas filter 222. Filter 800 comprises a lens 802, a blocking plate 804, ablocking plate 806 and a lens 808. Plate 806 has an aperture 810 locatedat a focal point 812 of lens 802. Plate 804 has a complementary aperture814 which allows the collimated light 230 to be directed throughaperture 810 while blocking unwanted uncollimated light 820. Aperture814 may be a pair of parallel slits or an annular aperture. In apreferred embodiment, the distance between the slits of aperture 814 isgreater than the diameter of aperture 810. The diameter of aperture 810is approximately equal to λ/(2*(NA)). For the alternative annular shapedaperture, the inner diameter of the annular slit should be greater thanthe diameter of aperture 810. In both cases, the outer edge 822 ofaperture 814 is located outside of beam 230. Blocking plates 804 and 806may be made of a metal sheet or may be made of a transparent substratehaving a light blocking coating with apertures 810 and 814 beinguncoated.

FIG. 25 shows a schematic diagram of an alternative filter 830 which maybe used as filter 222. Filter 830 comprises a beamsplitter 832 and aholographic plate 834. The coating on the holographic plate 834 is tunedto efficiently reflect collimated beam 230 while uncollimated beam 840is allowed to pass. The desired beam 230 is reflected from holographicplate 834 and returns to beamsplitter 832 where it is reflected towardsbeamsplitter 224.

FIG. 26 is a schematic diagram which shows how holographic plate 834 ismade. A collimated laser beam 850 having approximately the samewavelength as laser 200 is split into two beams 852 and 854 at anamplitude beamsplitter 856. Beams 852 and 854 are directed by mirrors860 and 862, respectively, and fall upon hologram plate 834 fromopposite directions perpendicular to the surface of plate 834. Areflective hologram is recorded by the interference of beams 852 and854. The holographic coating may be made of a dichromated gel orphotopolymer material.

Filters 222 of the present invention have been shown in FIG. 4 to belocated in the path of beam 220. However, one or more filters can belocated in the separate paths of servo beam 230 or the data beam 236.

While the preferred embodiments of the present invention have beenillustrated in detail, it should be apparent that modifications andadaptations to those embodiments may occur to one skilled in the artwithout departing from the scope of the present invention as set forthin the following claims.

We claim:
 1. An optical data storage medium comprising:a first lighttransmissive member consisting of a nonmetallic material and having anoncoated first data surface, the first data surface having marks whichare optically detectable, the marks being integral to the first lighttransmissive member; a second member having a second data surface, thesecond data surface having marks which are optically detectable, themarks being integral to the second member; and a spacer means locatedbetween the first and second members for defining a space between thefirst and second data surfaces, the space containing a gas, such that alight beam is selectively focussed through the first light transmissivemember to one of the first and second data surfaces.
 2. The optical datastorage medium of claim 1, wherein the gas is air.
 3. The optical datastorage medium of claim 1, wherein the marks are grooves.
 4. The opticaldata storage medium of claim 1, wherein the marks are inverse grooves.5. The optical data storage medium of claim 1, wherein the marks on thefirst data surface are grooves and the marks on the second data surfaceare inverse grooves.
 6. The optical data storage medium of claim 1,wherein the marks on the first data surface are inverse grooves and themarks on the second data surface are grooves.
 7. The optical datastorage medium of claim 1, wherein the marks are pits.
 8. The opticaldata storage medium of claim 1, wherein the marks are inverse pits. 9.The optical data storage medium of claim 1, wherein the marks on thefirst data surface are pits and the marks on the second data surface areinverse pits.
 10. The optical data storage medium of claim 1, whereinthe marks on the first data surface are inverse pits and the marks onthe second data surface are pits.
 11. The optical data storage medium ofclaim 1, wherein the first data surface is an uncoated ROM surface. 12.The optical data storage medium of claim 1, wherein the second datasurface is a ROM surface.
 13. The optical data storage medium of claim1, wherein the marks are tracking marks.
 14. The optical data storagemedium of claim 1, wherein the marks are data marks.
 15. The opticaldata storage medium of claim 1, further including at least oneadditional data surface.
 16. The medium of claim 1, wherein the firstlight transmissive member is comprised of glass.
 17. The medium of claim1, wherein the first light transmissive member is comprised of a polymermaterial.
 18. The medium of claim 1, wherein the second member consistsof a light transmissive nonmetallic material.
 19. The medium of claim 1,wherein the second member is comprised of glass.
 20. The medium of claim1, wherein the second member is comprised of a polymer material.
 21. Themedium of claim 1, wherein the spacer means comprises a first and asecond annular members.
 22. The medium of claim 1, wherein the secondmember is light transmissive and has a third data surface locatedopposite the second data surface, and further comprising a third memberhaving a fourth data surface, and a second spacer means for defining aspace between the third and fourth data surfaces, the space containing agas, the third and fourth data surfaces having marks which are opticallydetectable.
 23. An optical data storage system comprising:a light sourcefor producing a light beam; an optical medium comprising a first lighttransmissive substrate having a first light transmissive data surfacefor storing recorded data, the first data surface having marks which areoptically detectable; a second substrate having a second data surfacefor storing recorded data, the second data surface having marks whichare optically detectable; a spacer means located between the first andsecond substrates for defining a space between the first and second datasurfaces, the space containing a gas such that the light beam isselectively focussed through the first light transmissive member to oneof the first and second data surfaces, the spacer means having a passageconnecting the space with the atmosphere outside the medium, the passageallowing the movement of gas between the space and the atmosphereoutside the medium and the passage blocking the movement of solidparticles between the space and the atmosphere outside the medium; datasurfaces of the optical medium; and an optical data reception means forreceiving a reflected light beam from the optical medium and providing adata signal responsive thereto.
 24. The optical data storage medium ofclaim 23, further including a filter positioned across the passage. 25.The optical data storage medium of claim 23, wherein the gas is air. 26.The optical data storage medium of claim 23, wherein the marks aregrooves.
 27. The optical data storage medium of claim 23, wherein themarks are inverse grooves.
 28. The optical data storage medium of claim23, wherein the marks on the first data surface are grooves and themarks on the second data surface are inverse grooves.
 29. The opticaldata storage medium of claim 23, wherein the marks on the first datasurface are inverse grooves and the marks on the second data surface aregrooves.
 30. The optical data storage medium of claim 23, wherein themarts are pits.
 31. The optical data storage medium of claim 23, whereinthe marks are inverse pits.
 32. The optical data storage medium of claim23, wherein the marks on the first data surface are pits and the markson the second data surface are inverse pits.
 33. The optical datastorage medium of claim 28, wherein the marks on the first data surfaceare inverse pits and the marks on the second data surface are pits. 34.The optical data storage medium of claim 23, wherein at least one of thedata surfaces is a ROM surface.
 35. The optical data storage medium ofclaim 23, wherein at least one of the data surfaces has a WORM materialcoating.
 36. The optical data storage medium of claim 23, wherein atleast one of the data surfaces has a phase change material coating. 37.The optical data storage medium of claim 23, wherein at least one of thedata surfaces has a magneto-optic material coating.
 38. The optical datastorage medium of claim 23, wherein the first data surface has adielectric coating having an index of refraction greater than the indexof refraction of the first radiation transmissive member.
 39. Theoptical data storage medium of claim 23, wherein the first data surfacehas a dielectric coating having an index of refraction less than theindex of refraction of the first radiation transmissive member.
 40. Theoptical data storage medium of claim 23, wherein the marks are trackingmarks.
 41. The optical data storage medium of claim 23, wherein themarks are data marks.
 42. The optical data storage medium of claim 23,further including at least one additional data surface.
 43. The mediumof claim 23, wherein the first light transmissive member is comprised ofglass.
 44. The medium of claim 23, wherein the first light transmissivemember is comprised of a polymer material.
 45. The medium of claim 23,wherein the second member consists of a light transmissive nonmetallicmaterial.
 46. The medium of claim 23, wherein the second member iscomprised of glass.
 47. The medium of claim 23, wherein the secondmember is comprised of a polymer material.
 48. The medium of claim 23,wherein the spacer means comprises a first and a second annular members.49. The medium of claim 23, wherein the second member is lighttransmissive and has a third data surface located opposite the seconddata surface, and further comprising a third member having a fourth datasurface, and a second spacer means for defining a space between thethird and fourth data surfaces, the space containing a gas, the thirdand fourth data surfaces having marks which are optically detectable.50. An optical data storage system comprising:a light source forproducing a light beam; an optical medium comprising a first lighttransmissive member consisting of a nonmetallic material and having anoncoated first data surface, the first data surface having marks whichare optically detectable, the marks being integral to the first lighttransmissive member; a second member having a second data surface, thesecond data surface having marks which are optically detectable, themarks being integral to the second member; a spacer means locatedbetween the first and second members for defining a space between thefirst and second data surfaces, the space containing a gas, such thatthe light beam is selectively focussed through the first radiationtransmissive member to one of the first and second data surfaces; anoptical transmission means for directing the light beam from the lightsource to one of the data surfaces of the optical medium; and an opticalreception means for receiving a reflected light beam from the opticalmedium and providing a data signal responsive thereto.
 51. The medium ofclaim 50, wherein the first light transmissive member is comprised ofglass.
 52. The medium of claim 50, wherein the first light transmissivemember is comprised of a polymer material.
 53. The medium of claim 50,wherein the second member consists of a light transmissive nonmetallicmaterial.
 54. The medium of claim 50, wherein the second member iscomprised of glass.
 55. The medium of claim 50, wherein the secondmember is comprised of a polymer material.
 56. The medium of claim 50,wherein the spacer means comprises a first and a second annular members.57. The optical data storage medium of claim 50, wherein the gas is air.58. The optical data storage medium of claim 50, wherein the marks aregrooves.
 59. The optical data storage medium of claim 50, wherein themarks are inverse grooves.
 60. The optical data storage medium of claim50, wherein the marks on the first data surface are grooves and themarks on the second data surface are inverse grooves.
 61. The opticaldata storage medium of claim 50, wherein the marks on the first datasurface are inverse grooves and the marks on the second data surface aregrooves.
 62. The optical data storage medium of claim 50, wherein themarks are pits.
 63. The optical data storage medium of claim 50, whereinthe marks are inverse pits.
 64. The optical data storage medium of claim50, wherein the marks on the first data surface are pits and the markson the second data surface are inverse pits.
 65. The optical datastorage medium of claim 50, wherein the marks on the first data surfaceare inverse pits and the marks on the second data surface are pits. 66.The optical data storage medium of claim 50, wherein the first datasurface is an uncoated ROM surface.
 67. The optical data storage mediumof claim 50, wherein the second data surface is a ROM surface.
 68. Theoptical data storage medium of claim 50, wherein the marks are trackingmarks.
 69. The optical data storage medium of claim 50, wherein themarks are data marks.
 70. The optical data storage medium of claim 50,further including at least one additional data surface.
 71. The mediumof claim 50, wherein the second member is light transmissive and has athird data surface located opposite the second data surface, and furthercomprising a third member having a fourth data surface, and a secondspacer means for defining a space between the third and fourth datasurfaces, the space containing a gas, the third and fourth data surfaceshaving marks which are optically detectable.
 72. An optical data storagemedium comprising:a first light transmissive substrate having a firstlight transmissive data surface for storing recorded data, the firstdata surface having marks which are optically detectable; a secondsubstrate having a second data surface for storing recorded data, thesecond data surface having marks which are optically detectable; and aspacer means located between the first and second substrates fordefining a space between the first and second data surfaces, the spacecontaining a gas such that a light beam is selctively focussed throughthe first light transmissive member to one of the first and second datasurfaces, the spacer means having a passage connecting the space withthe atmosphere outside the medium, the passage allowing the movement ofgas between the space and the atmosphere outside the medium and thepassage blocking the movement of solid particles between the space andthe atmosphere outside the medium.
 73. The optical data storage mediumof claim 72, wherein the gas is air.
 74. The optical data storage mediumof claim 72, wherein the marks are grooves.
 75. The optical data storagemedium of claim 72, wherein the marks are inverse grooves.
 76. Theoptical data storage medium of claim 72, wherein the marks on the firstdata surface are grooves and the marks on the second data surface areinverse grooves.
 77. The optical data storage medium of claim 72,wherein the marks on the first data surface are inverse grooves and themarks on the second data surface are grooves.
 78. The optical datastorage medium of claim 72, wherein the marks are pits.
 79. The opticaldata storage medium of claim 72, wherein the marks are inverse pits. 80.The optical data storage medium of claim 71, wherein the marks on thefirst data surface are pies and the marks on the second data surface areinverse pits.
 81. The optical data storage medium of claim 72, whereinthe marks on the first data surface are inverse pits and the marks onthe second data surface are pits.
 82. The optical data storage medium ofclaim 72, wherein at least one of the data surfaces is a ROM surface.83. The optical data storage medium of claim 72, wherein at least one ofthe data surfaces has a WORM material coating.
 84. The optical datastorage medium of claim 72, wherein at least one of the data surfaceshas a phase change material coating.
 85. The optical data storage mediumof claim 72, wherein at least one of the data surfaces has amagnetic-optic material coating.
 86. The optical data storage medium ofclaim 72, wherein the first data surface has a dielectric coating havingan index of refraction greater than the index of refraction of the firstradiation transmissive member.
 87. The optical data storage medium ofclaim 72, wherein the first data surface has a dielectric coating havingan index of refraction less than the index of refraction of the firstradiation transmissive member.
 88. The optical data storage medium ofclaim 72, wherein the marks are tracking marks.
 89. The optical datastorage medium of claim 72, wherein the marks are data marks.
 90. Theoptical data storage medium of claim 72, further including at least oneadditional data surface.
 91. The optical data storage medium of claim72, further including a filter positioned across the passage.
 92. Themedium of claim 72, wherein the first light transmissive member iscomprised of glass.
 93. The medium of claim 72, wherein the first lighttransmissive member is comprised of a polymer material.
 94. The mediumof claim 72, wherein the second member consists of a light transmissivenonmetallic material.
 95. The medium of claim 72, wherein the secondmember is comprised of glass.
 96. The medium of claim 72, wherein thesecond member is comprised of a polymer material.
 97. The medium ofclaim 72, wherein the spacer means comprises a first and a secondannular members.
 98. The medium of claim 72, wherein the second memberis light transmissive and has a third data surface located opposite thesecond data surface, and further comprising a third member having afourth data surface, and a second spacer means for defining a spacebetween the third and fourth data surfaces, the space containing a gas,the third and fourth data surfaces having marks which are opticallydetectable.